diff --git a/src/corelib/kernel/qmetatype.cpp b/src/corelib/kernel/qmetatype.cpp
index 65745bf06a..41b991580f 100644
--- a/src/corelib/kernel/qmetatype.cpp
+++ b/src/corelib/kernel/qmetatype.cpp
@@ -1560,9 +1560,11 @@ public:
     bool insertIfNotContains(Key k, const T &f)
     {
         const QWriteLocker locker(&lock);
-        if (map.contains(k))
+        const qsizetype oldSize = map.size();
+        auto &e = map[k];
+        if (map.size() == oldSize) // already present
             return false;
-        map.insert(k, f);
+        e = f;
         return true;
     }